In an injection-molding machine, multi-stage pressure control is performed to produce good-quality moldings. For example, a pressure hold process is divided into a plurality of pressure hold stages, and target hold pressures for the individual pressure hold stages are set to values different from one another, so that the hold pressures are subjected to multi-stage control. To effect the multi-stage hold pressure control, in a motor-operated injection-molding machine, a torque limit value (target hold pressure) for restricting the output torque of a servomotor for axially driving an injection screw is successively changed with the lapse of time from an instant at which pressure hold is started, so that the hold pressure is subjected to open-loop control. Alternatively, the hold pressure is subjected to closed-loop control (feedback control) in accordance with the difference between the torque limit value thus changed in succession and a detected value of the hold pressure actually acting on resin.
However, the multi-stage pressure control of the injection-molding machine has a problem such that the actual pressure overshoots or undershoots the target pressure at the time of switching the target pressure, due to various factors, including inertia of the screw, inertia of the servomotor, incompatibility of the response output characteristic of the servomotor, and incompatibility of the control gain of a closed-loop control system. The incompatibility of the control gain can be eliminated by adjusting, for example, the proportional, differential, and integral parameters of a PID control section of the control system, without changing the arrangement of the control system. In any control method, however, it is difficult to eliminate unsatisfactory responsiveness attributable to the servomotor or the like. More specifically, in the case of mounting the injection-molding machine with a motor having a response output characteristic compatible with the type and pressure control pattern of the injection-molding machine, or in replacing a once mounted motor with another motor, so as to improve the performance for the actual pressure to follow up the target pressure, it is necessary to provide motors of various specifications and to perform replacement work for the motors. Accordingly, the production and running costs of the injection-molding machine increase considerably. In the case of setting the pressure control pattern agreeable to the motor characteristic, moreover, feasible pressure control patterns are limited, so that there are restrictions on the pressure control. Thus, none of the aforementioned countermeasures are practical for the manufacture of high-quality, low-priced moldings.